Why does everyone like DOF preview buttons?

Even on cameras that I've owned that have them, I never use them. Personally I find them pretty useless. Yet I see lots of discussions on different bodies here where people say "only thing wrong with it is it lacks a DOF preview."

So I'm just wondering how people put them to practical use.

I might add that I've shot with a lot of rangefinders in my days (so no DOF buttons) and that I shoot a lot of wide angle work (often at high fstops) so I'm pretty aware of, and quite like, a lot of depth of field.

I guess my main problem with them has been, in general (and maybe its just the ones I've used) they darken parts of the focusing screen .. and I can't really tell anything about the DOF beyond what I already know in my head -- or what I can tell by looking at the DOF scale on the lens (which I'll admit I rarely ever did.) I guess I already have a general idea what 5.6 or 1.8 is going to look like at a given focal point .... so I haven't ever found the DOF preview to be any more accurate than my imagination.

Well, it would darken the focusing screen, since it is stopping down the diaphragm. Usually, going from f1.8 to f5.6, I ignore the DOF button. I usually will only use it if it is a very bright day and I am using a higher fstop.

Yeah, I do wish there was a way to see a DOF preview without it going dim. I find them more useful at higher fstops because I am usually trying to reach out and get something distant in focus. If I use it and then squint, I can usually tell if its in focus or not.

I find them most useful when I am trying to evaluate how out of focus different parts of the foreground and background are.

A highlight in the background or foreground which is pleasingly blurred at the maximum aperture of f/2.8 may still be sufficiently blurred at f/4.0 but by f/5.6 or f/8.0 it may show sufficient contrast and detail to distract from the rest of the image. The depth of field preview helps me evaluate this.

Matt

“Photography is a complex and fluid medium, and its many factors are not applied in simple sequence. Rather, the process may be likened to the art of the juggler in keeping many balls in the air at one time!”

Ansel Adams, from the introduction to The Negative - The New Ansel Adams Photography Series / Book 2